It is important that wireless signals transmitted by a mobile device arrive at a base station or access point during a predetermined reception window corresponding to the base station's frame timing. Without proper alignment with the reception window, the mobile device's transmissions may interfere with signals in an adjacent time frame, sub-frame, or time slot. Furthermore, improper time alignment at handover may cause the signals to arrive outside the base station's reception window. To capture the desired signals from the mobile device at the base station, a much longer reception window will have to be implemented. Longer reception window increases complexity of the receiver at the base station since they require more hardware and processing ability. Due to the mobility of the mobile device, the distance between a base station and the mobile device varies. This in turn varies the propagation delay between the base station and the mobile device. Thus, to ensure that a mobile device's transmissions arrive at the base station within the correct reception window, the network may regularly monitor the propagation delay of the mobile device, and signal the appropriate time offset or transmission time corresponding to the propagation delay to the mobile device. The mobile device uses the received offset or transmission time such that its transmissions arrive at the base station at a desired time.
During a handover of the mobile device from a serving cell to a target cell, the mobile device needs to adjust its transmit timing to account for differences in the propagation delay from the mobile device to the target cell and the serving cell. The failure to make the timing adjustment may produce interference and/or prolong synchronization and handover. For example, during handover in LTE networks, the mobile device uses a random access procedure to access the target base station and acquire physical layer synchronization with the target base station. If the mobile device accesses the target base station during handover at a transmission time that does not match up with the frame timing of the target base station, the handover signals received at the target base station may interfere with adjacent slots/frames, may cause a high load on a Random Access Channel (RACH) due to retransmission(s), and/or may cause longer handover interruptions due to longer synchronization times. The RACH is also used by mobile devices for initial access, and therefore, excessive RACH retransmissions at handover may increase RACH load and collisions. Thus, at high loads there is risk that if transmission timing is not adjusted, the RACH transmission may become unstable.
One potential solution to match the mobile device transmission time with the desired target cell frame timing is to determine and apply a propagation-related offset to the mobile device's transmission time during handover. For example, the target base station may calculate the uplink propagation delay relative to the mobile device and signal the corresponding offset to the mobile device. Based on the received offset, the mobile device adjusts (e.g., advances) its next transmission time. This approach, however, may delay uplink synchronization, especially in large cells. In LTE, where handover access takes place on the RACH, the mobile device's initial transmission to the target base station on the RACH is not compensated for the propagation delay offset relative to the target base station, and thus may cause a high load on the RACH and eventually long handover interruption times.
Another potential solution is to estimate the propagation-related offset at the target base station before handover. In this case, the target base station transmits a reference signal to the mobile device before handover, and the mobile device transmits a response. Based on the response, the target base station measures the propagation delay and communicates the corresponding offset to the serving base station, which in turn signals the offset to the mobile device before the handover begins, e.g., via a handover command. The advantage of this solution is that the mobile device has proper timing relative to the target base station during handover. However, this solution requires some timing estimation at the target base station before handover. Further, signaling the offset undesirably imposes signaling overhead on the transport network between the serving and target base stations.
Thus, there remains a need for alternative methods for aligning signal transmissions between a mobile device and a target base station during handover.